Photoacoustic gas analyzer for determining species concentrations using intensity modulation

Abstract

A photoacoustic gas analyzer including a gas chamber to receive a gas sample, a radiation source to emit an electromagnetic radiation adapted to excite N different types of gas molecules in the gas sample, the concentrations of which are to be determined, an acoustic-wave sensor to detect acoustic waves generated by the irradiated gas, and a control unit. The control unit controls the radiation source to emit electromagnetic radiation with a time-varying intensity and to modulate the frequency at which the intensity is varied with a modulation signal having at least N different values, to receive from the acoustic-wave sensor signals indicative of acoustic waves generated by the irradiated gas, to determine at least N mutually different signal amplitudes each associated with a respective N mutually different frequencies at which the intensity of the emitted electromagnetic radiation is varied, and to determine the concentrations of the N different gas types.

What is claimed is: 
     
       1. A photoacoustic gas analyzer, comprising:
 a gas chamber configured to receive a gas to be analyzed therein; a radiation source configured to emit into the gas chamber electromagnetic radiation with a time-varying intensity adapted to selectively excite gas molecules of N mutually different gas types the concentrations of which are to be determined in the gas received in the gas chamber, thereby generating acoustic waves; 
 an acoustic-wave sensor configured to detect acoustic waves generated by the electromagnetic radiation emitted by the radiation source into the gas to be analyzed; and 
 a control unit operatively connected to the radiation source and the acoustic-wave sensor, wherein the control unit is configured:
 to control the radiation source to emit electromagnetic radiation with a time-varying intensity, wherein a frequency at which the time-varying intensity is varied is based on a modulation signal taking on at least N mutually different values; 
 to receive from the acoustic-wave sensor signals indicative of detected acoustic waves generated by the electromagnetic radiation emitted by the radiation source into the gas to be analyzed; 
 to determine at least N mutually different signal amplitudes associated with respective N mutually different values at which the time-varying intensity of the emitted electromagnetic radiation is varied, wherein the N mutually different values comprise N mutually different frequencies; and 
 to determine from the determined at least N mutually different signal amplitudes the concentrations of the N mutually different gas types, 
 wherein the control unit is configured to determine the concentrations of the N mutually different gas types based on the at least N mutually different signal amplitudes, each signal amplitude associated with one of the respective N mutually different frequencies at which the time-varying intensity of the emitted electromagnetic radiation is varied, and each signal amplitude comprising signal components, I 1  to I N , associated with the N mutually different gas types and indicative of the respective proportional concentrations of the N mutually different gas types, and further based on proportionality factors, wherein the proportionality factors correspond to the N mutually different gas types, the proportionality factors determined based on calibrations using samples with known concentrations of the N mutually different gas types. 
 
 
     
     
       2. The photoacoustic gas analyzer of  claim 1 ,
 wherein the modulation signal is at least partially strictly monotonically increasing and/or at least partially strictly monotonically decreasing. 
 
     
     
       3. The photoacoustic gas analyzer of  claim 2 ,
 wherein the modulation signal is at least partially a sinusoidal signal, a triangle signal, or a sawtooth signal. 
 
     
     
       4. The photoacoustic gas analyzer of  claim 1 ,
 wherein the modulation signal is at least partially a staircase signal. 
 
     
     
       5. The photoacoustic gas analyzer of  claim 1 ,
 further comprising a filter configured to selectively transmit electromagnetic radiation of a predetermined energy range emitted by the radiation source into the gas chamber, wherein the predetermined energy range comprises excitation energies of the molecules of each of the N gas types the concentrations of which in the gas chamber are to be determined. 
 
     
     
       6. The photoacoustic gas analyzer of  claim 5 ,
 wherein the filter has fixed transmission characteristics. 
 
     
     
       7. The photoacoustic gas analyzer of  claim 1 ,
 wherein the acoustic-wave sensor is positioned inside of the gas chamber. 
 
     
     
       8. The photoacoustic gas analyzer of  claim 1 ,
 wherein the acoustic-wave sensor is positioned inside of a reference-gas chamber gas-tightly separated from the gas chamber by a window transparent for electromagnetic radiation emitted by the radiation source. 
 
     
     
       9. The photoacoustic gas analyzer of  claim 1 ,
 wherein the radiation source is configured as or comprises at least one selected from: a photodiode, a laser, a black-body radiator, and a gray-body radiator. 
 
     
     
       10. The photoacoustic gas analyzer of  claim 9 ,
 wherein the radiation source is configured as or comprises a black-body radiator or a gray-body radiator configured as an electrically heatable body. 
 
     
     
       11. The photoacoustic gas analyzer of  claim 1 ,
 wherein the gas chamber is delimited by a reflector configured to reflect electromagnetic radiation emitted by the radiation source. 
 
     
     
       12. The photoacoustic gas analyzer of  claim 1 ,
 wherein the control unit is further configured:
 to determine a set of local maximums of each of the acoustic-wave sensor signals; 
 to determine the N mutually different signal amplitudes based on the set of local maximums of each of the acoustic-wave sensor signals and corresponding timings of the set of local maximums; 
 to assign the N mutually different signal amplitudes to a respective one of the N mutually different frequencies; and 
 to determine partial amplitudes I 1  to I N  associated with the N mutually different gas types and indicative of the concentrations of the N mutually different gas types for each of the N mutually different signal amplitudes. 
 
 
     
     
       13. The photoacoustic gas analyzer of  claim 12 ,
 wherein the frequency at which the time-varying intensity of the electromagnetic radiation is varied is a function of time. 
 
     
     
       14. The photoacoustic gas analyzer of  claim 12 ,
 wherein the modulation signal has a linear relationship between frequency and time. 
 
     
     
       15. A method of operating a photoacoustic gas analyzer, wherein the photoacoustic gas analyzer comprises:
 a gas chamber configured to receive a gas to be analyzed therein, 
 a radiation source configured to emit into the gas chamber electromagnetic radiation with a time-varying intensity adapted to selectively excite gas molecules of N mutually different gas types the concentrations of which are to be determined in the gas received in the gas chamber, thereby generating acoustic waves; 
 an acoustic-wave sensor configured to detect acoustic waves generated by the electromagnetic radiation emitted by the radiation source into the gas to be analyzed; and 
 a control unit operatively connected to the radiation source and the acoustic-wave sensor, wherein the control unit is configured: 
 to control the radiation source to emit electromagnetic radiation with a time-varying intensity, wherein a frequency at which the time-varying intensity is varied is based on a modulation signal taking on at least N mutually different values; 
 to receive from the acoustic-wave sensor signals indicative of detected acoustic waves generated by the electromagnetic radiation emitted by the radiation source into the gas to be analyzed; 
 to determine at least N mutually different signal amplitudes associated with respective N mutually different frequencies at which the time-varying intensity of the emitted electromagnetic radiation is varied; and 
 to determine from the determined signal amplitudes the concentrations of the N mutually different gas types, 
 wherein the method comprises:
 controlling the radiation source to emit into the gas chamber electromagnetic radiation with a time-varying intensity, wherein the frequency at which the time-varying intensity is varied is modulated with the modulation signal; 
 receiving from the acoustic-wave sensor signals indicative of detected acoustic waves generated by the electromagnetic radiation emitted by the radiation source into the gas to be analyzed; 
 determining at least N mutually different signal amplitudes associated with respective N mutually different values at which the time-varying intensity of the emitted electromagnetic radiation is varied, wherein the N mutually different values comprise N mutually different frequencies; and determining from the determined at least N mutually different signal amplitudes the concentrations of the N mutually different gas types, 
 wherein the concentrations of the N mutually different gas types is determined based on the at least N mutually different signal amplitudes, each signal amplitude associated with one of the respective N mutually different frequencies at which the time-varying intensity of the emitted electromagnetic radiation is varied, and each signal amplitude comprising signal components, I 1  to I N , associated with the N mutually different gas types and indicative of the respective proportional concentrations of the N mutually different gas types, and further based on proportionality factors, wherein the proportionality factors correspond to the N mutually different gas types, the proportionality factors determined based on calibrations using samples with known concentrations of the N mutually different gas types at the N mutually different frequencies and the at least N mutually different signal amplitudes. 
 
 
     
     
       16. The method of  claim 15 ,
 wherein the modulation signal is at least partially strictly monotonically increasing and/or at least partially strictly monotonically decreasing. 
 
     
     
       17. The method of  claim 16 ,
 wherein the modulation signal is at least partially a sinusoidal signal, a triangle signal, or a sawtooth signal. 
 
     
     
       18. The method of  claim 15 ,
 wherein the modulation signal is at least partially a staircase signal. 
 
     
     
       19. The method of  claim 15 , further comprising:
 determining a set of local maximums of each of the acoustic-wave sensor signals; 
 determining the N mutually different signal amplitudes based on the set of local maximums of each of the acoustic-wave sensor signals and corresponding timings of the set of local maximums; 
 assigning the N mutually different signal amplitudes to a respective one of the N mutually different frequencies; and 
 determining partial amplitudes I 1  to I N  associated with the N mutually different gas types and indicative of the concentrations of the N mutually different gas types for each of the N mutually different signal amplitudes. 
 
     
     
       20. The method of  claim 19 ,
 wherein the frequency at which the time-varying intensity of the electromagnetic radiation is varied is a function of time. 
 
     
     
       21. The method of  claim 19 ,
 wherein the modulation signal has a linear relationship between frequency and time.